1. Technical Field
The present invention relates to tape storage systems in general. More particularly, the present invention relates to a method and apparatus for providing longitudinal position (LPOS) data in a tape storage system. Still more particularly, the present invention relates to a method and apparatus for providing error correction capability to LPOS words in a tape storage system.
2. Description of Related Art
Tape storage systems remain to be the most efficient and cost-effective means for providing data backup because no other storage technology offers the same low cost and high capacity combined advantage. In addition, tape storage systems have been proven to be very reliable.
By combining the advantages of linear multi-channel bidirectional tape formats in common usage, Linear Tape-Open (LTO) technology has been developed to maximize capacity and performance of tape storage systems. LTO tapes use a tape format that has longitudinally pre-written servo tracks. The servo tracks provide a timing-based track-following position error scheme. The servo tracks contain a repeated pattern of recorded flux transitions that occur in grouped bursts of 5, 5, 4 and 4 transitions. The timing between the sets of five-bursts and between sets of four-bursts provides the position information for track following. Additionally, the individual transitions within the five-bursts are phase-shifted in a manner that encodes longitudinal position information (LPOS) into the servo tracks.
The LPOS information are used to keep track of the longitudinal position of data records written onto or read from a tape, and are used to locate those data records when the reading or writing process temporarily stops. By detecting the phase-encoded LPOS information, a tape storage system is able to determine the tape position relative to landmarks lengthwise down a tape. The LPOS locations of data files on tape are also stored in the volume control data for use to locate the data files during a later tape cartridge load for reading, or for write-appending new files onto the end of the last file written to the tape. The LPOS data are used as the primary positional information for the tape storage servo control system to determine the starting and stopping of a tape, and to back-hitch the tape in order to position the read-write heads at the beginning of a data record at the required velocity and track position that allows the start of a new data transfer operation.
LPOS data typically cannot tolerate any errors. But if a tape drive is reduced to a single channel because other servo heads have been smeared or shorted, a single bit error on that channel can cause a Stop Write condition. Thus, LPOS data have to able to tolerate some level of errors such that the good servo head can continue to operate after an occurrence of an error.
One known solution to the above-mentioned problem is to append Reed-Solomon parity symbols to an LPOS word, but it would increase the length of the LPOS word, and it would cause problems in synchronizing to the LPOS word since Reed-Solomon words are not base-14 as required by the LPOS format in order to allow for synchronization. Another solution is to add a base-14 checksum, but again it would increase the length of the LPOS word, and it only provides error detection but not error correction.
Consequently, it would be desirable to provide an improved method and apparatus for providing error correction capability to LPOS words.